CATALYSIS OF THE ELECTROCHEMICAL REDUCTION OF CARBON-DIOXIDE BY IRON(0) PORPHYRINS - SYNERGISTIC EFFECT OF LEWIS-ACID CATIONS

Addition of Lewis acid cations such as Mg2+, Ca2+, Ba2+, Li+ and Na+ c onsiderably improves the catalysis of CO2 reduction by iron(0) tetraph enylporphyrins, in terms of both catalytic efficiency and lifetime of the catalyst. Carbon monoxide is the main product, while formate is fo rmed to a lesser extent (30% with Mg2+ and only 10% with Li+). Cyclic voltammetry indicates that the order of reactivity of these Lewis acid synergists is Mg2+ congruent to Ca2+ > Ba2+ > Li+ > Na+. Systematic a nalysis of the kinetics allows the proposal of a mechanism for the eff ect of both the divalent and monovalent cations. In many instances, th e assisted catalysis is so strong that the current is affected by prod uct inhibition as observed previously with Bronsted acid synergists. S elf-inhibition is more irreversible in the present case and required a different approach to be analyzed. The importance of push-pull mechan isms in the catalysis of CO2 reduction is thus confirmed; electrons ar e pushed into the CO2 molecule by the electron-rich catalyst and the c leavage of one of the C-O bonds is helped by the presence of an electr on deficient synergist.